As for the size and length of occupation, MacArthur's experience in Japan answers both questions. Truman told Congress that 400,000 troops would be needed. MacArthur said only 200,000 were needed, and he was right. Since Japan then was about three times the size of Iraq now, about 60,000 troops will be required for post-war Iraq.
As for the time required, MacArthur moved Japan from a dictatorship to a fully funtional democratic republic in two years. The major withdrawal of American troops began then, and was completed in six and one half years. It ended at 60,000, which would be about 18,000 in Iraq. (Far less, I note, than the US currently has in Germany where they are neither useful nor properly welcome.)
As for the ethnic diversity in Iraq, which Japan did not have, that is the whole point of Jonah Goldberg's column. So, exactly as I say, combine the history of post-war Japan with 500 years of Swiss history. Throw in a dollop of Philadelphia, circa 1787, and there you have it. (BTW, Germany was not a nation but a collection of tribes, until shortly before 1900.)
History, as often happens, provides ample answers to a current problem. It just requires looking in the right places and asking the correct questions. Jonah was exactly right about the SINGLE subject his column addressed, the accommodation of ethnic diversity in Iraq. I am right about the other aspect discussed above.
Congressman Billybob